Signal System Timings
To maximize traffic flow along a corridor, closely spaced signals are inter-connected, creating a coordinated signal system. Coordination refers to the timing of the signals so that a platoon of vehicles traveling on a street arrives at a succession of green lights and proceeds through multiple intersections without stopping. Coordination facilitates smooth traffic flow (progressed movement) along a street. Typically, the busiest traffic movements are given priority in traffic coordination as the goal of coordination is to get the greatest number of vehicles through the system with the fewest stops.
Depending on the route, the master cycle length could vary from 60 to 120 seconds. Generally, the busier and the bigger the intersection, the longer the required cycle length. Common practice is to coordinate signals less than one-half mile apart on major streets and highways.
Though traffic signal coordination is a major element in the effort to maximize traffic flow and improve the performance of the street system, not all streets warrant coordination. A street is selected for coordination if it carries a certain amount of traffic along the major route during peak hours. In most cases, coordination is active in the morning and late afternoon on weekdays. Outside of these hours, the individual signals operate on a "first-come-first-served" or traffic activated basis. Coordinated signal systems result in fewer stops on the major street. Fewer stops will lead to improved traffic flow, fewer crashes, more uniform vehicle speeds, as well as reduced fuel consumption, pollutant emissions, and operating costs. The benefits of coordinated signal timing are well known.
Advantages of good traffic progression:
- Improved traffic flow
- Fewer crashes
- More uniform vehicle speeds
- Reduced fuel consumption, pollutants emissions, and operating costs
There are various computerized traffic simulation models to design and develop proper coordinated signal timings. Some of the programs in current use include Synchro and TRANSYT-7F.
Traffic signals can be programmed to have different signal timing plans, depending on the time of day. Management of the coordination should be done on an on-going basis. The efficiency of a coordinated signal system should be evaluated and maintenance improvements (such as timing adjustments to the individual signals) implemented to obtain maximum performance.